Melt the butter in a large saucepan over low heat and add marshmallows. Stir slowly until the marshmallows begin to melt. Then add in the cake mix one spoonful at a time until it's combined. Stir in the cereal so it is completely coated with marshmallow mixture. Sprinkle in half of the sprinkles and mix. Press into a 9x13" baking pan using damp hands or wax paper and top with remaining sprinkles. Let sit for about 30 minutes before cutting.